This invention relates generally to radio frequency antennas and more particularly to array antennas which include annular slot-type stripline antenna elements.
As is known in the art, annular slot-type stripline antenna elements are useful in radio frequency antennas, as where such an antenna is to be substantially flush-mounted to a vehicle, such as an aircraft or a missile. One such annular slot-type stripline antenna element is described in U.S. Pat. No. 3,665,480, Annular Slot Antenna With Stripline Feed, Inventor Matthew Fassett, issued May 23, 1972 and assigned to the same assignee as the present invention. As discussed therein, the antenna element includes a pair of parallel conductive plates formed on opposiite faces of a dielectric support structure, one of which has formed therein a generally annular radiating slot of substantially uniform width, and a feed element disposed between the parallel plates and extending radially into the central region of the annular slot for feeding electromagnetic energy into such slot. The electromagnetic energy has an electrid field component, the magnitude of which varies cosinusoidally with position from the feed about the circumference of the slot. A condition of resonance occurs when the circumference of the slot is approximately one wavelength. The phase of the electric field induced in the slot will then vary uniformly from 0.degree. to 360.degree. around the circumference of the slot which thereby produces a radiated field having its maximum intensity along the axis which is normal to the surface of the slot. In practice, for a slot with a finite width it has been found that the inner circumference of the slot should be approximately ten percent greater than the operating wavelength.
As described in the above-referenced U.S. patent, the antenna therein disclosed has a bandwidth in the order of 10%. Therefore, while such antenna has been found adequate in many applications, it is, however, frequently desirable to provide an antenna which is adapted to operate at frequencies which are separated by greater than 10%, say where one frequency is one-third greater than a second frequency.
As is further known in the art, in an array antenna the spacing, "a", between the centers of adjacent antenna elements must be a .ltoreq. (1 -1/N) .lambda..sub.H /(1+sin .theta.)=K.lambda..sub.H, (where N is the number of antenna elements along a scan axis of the array antenna, .lambda..sub.H is the wavelength of the highest operating frequency of the array antenna, .theta. is the maximum angular deviation of the beam from the boresight axis of the array antenna, and K is a proportionality constant, (1-1/N)/(1+sin .theta.) in order to obtain satisfactory grating lobe reduction. Therefore, if a first annular slot antenna element of the type discussed above were provided to accommodate the higher frequency and if it is desired to have the array operate at a second, lower frequency by means of a second, separately fed, concentric annular slot of the above type, it follows that the circumference of such second slot would be =1.1.lambda..sub.L (where .lambda..sub.L is the wavelength of such lower frequency) and the diameter, S, of such second slot would be 1.1.lambda..sub.L /.pi.. Therefore, in order to satisfy the requirement for grating lobes "a" .ltoreq.K.lambda..sub.H and the physical space requirement (i.e. no overlapping) for the second slot, the diameter of the second slot, S, must be less than (or equal to) "a", i.e. S .ltoreq. "a" or 1.1.lambda..sub.L /.pi..ltoreq.K.lambda..sub.H. Therefore, for example, for an array antenna where .theta. is 80.degree. and N=6, K=0.42, and ##EQU1## However, because of the physical space required for the feed elements and because the circumference of the radiating slot is about 10% greater than .lambda..sub.L as discussed above, and considering that the slots have finite widths, the maximum ratio of .lambda..sub.L /.lambda..sub.H in a practical case is less than 1.2. Consequently, considering also that space must be allowed for both feeds, the above described approach will not provide an array antenna of such type where such antenna is to operate at frequencies separated by over twenty percent.